(1) Field of the Invention
The present invention relates to a semiconductor laser and a manufacturing method thereof, and relates to a DFB (Distributed Feed Back type) laser or a DBR (Distributed Bragg Reflector type) laser which operates in a single wavelength, for example, as the light source for a DWDM (Dense Wavelength Division Multiplexing) optical-fiber transmission system.
(2) Description of Related Art
Recently, as the demand for the Internet has increased explosively, an ultra-higher speed and a larger capacity in an optical-communication/optical-transmission system has been actively pursued.
In particular, the larger capacity has been realized thanks to the significant development of the wavelength multiplexing optical-transmission technology. What supports this technology are a single wavelength laser or a tunable laser for stably providing light with a number of different wavelengths, and a fiber amplifier to amplify and relay collectively a large number of optical signals transmitting over an optical fiber. For example, the DFB laser which operates in a single wavelength is used as the light source of the transmitting side.
In the DWDM optical-transmission system, the channel is typically set at a frequency interval of 50 GHz or 100 GHz. Namely, converting this into wavelength, the channel is set at a wavelength interval of approximately 0.4 nm or approximately 0.8 nm. The deviation of the absolute wavelength of these channels needs to be set to ±0.05 nm or less. As the light source of such DWDM optical-transmission system, the DFB laser which oscillates stably in a single wavelength is mainly used.
Here, FIG. 21 is a view showing a typical structure of a DFB laser, and FIG. 22 is a cross sectional plan view along the alternate long and short dash line shown by A-A′ and the alternate long and short dash line shown by B-B′ of FIG. 21.
As shown in FIG. 21, the DFB laser is configured as including: an n-type doped InP substrate 1, an undoped GaInAsP guide layer 2 (with a thickness of 150 nm, and a composition wavelength of 1150 nm), a quantum well active layer 3 in which an undoped GaInAsP quantum well layer (with a thickness of 5.1 nm) and an undoped GaInAsP barrier layer (with a thickness of 10 nm, a composition wavelength of 1300 nm) are stacked repeatedly 7 times, a p-type doped InP electric-current constriction layer 4, an n-type doped InP electric-current constriction layer 5, a p-type doped InP cladding layer 6, a p-type doped GaInAs contact layer 7, a p-side electrode 8, an n-side electrode 9, and a diffraction grating 10 with a period of 242 nm, and a depth of 50 nm.
The oscillation wavelength λDFB of this DFB laser is expressed by the following equation, using a value neq inherent to a waveguide and referred to as the equivalent refractive index of this laser, and the period Λ of the diffraction grating formed along the waveguide (designated by the numeral 10 in FIG. 22).λDFB=2×neq×Λ
In the conventional DFB laser configured this way, in the step of designing the structure of the laser, the period Λ of the diffraction grating is determined in advance in accordance with the oscillation wavelength λDFB, and at the time of manufacturing the laser the diffraction grating is formed inside the semiconductor crystal (here, between the guide layer (waveguide) 2 extending in parallel with the active layer 3 and the n-type doped InP substrate 1). Namely, at the time of manufacturing the laser, the diffraction grating is formed in the course of the steps of stacking the semiconductor layers.
In addition, the semiconductor lasers having the diffraction grating are disclosed in Japanese Patent Laid-Open (Kokai) Hei 6-296063, Japanese Patent Laid-Open (Kokai) Hei 10-178232, and Japanese Patent Laid-Open (Kokai) 2003-152273, for example.